1. Field of the Invention
The present invention relates to precise, slow burning, pyrotechnic time delay compositions, and more particularly, to such compositions that are based upon environmentally friendly materials including boron carbide.
2. Background Art
Pyrotechnic delays provide reproducible time intervals between energetic events and are especially useful in munitions and military signals, including hand-held signals (HHS). While such delays have been replaced by very precise electronic time delays in advanced munitions, the simplicity and low cost of pyrotechnic delays is advantageous for mass produced and inexpensive munition components—such as, fuzes for hand grenades and delay elements for battlefield signal devices. Currently, such devices typically use relatively slow burning delay compositions which include a tungsten based delay formulation (W/BaCrO4/KClO4/diatomaceous earth), a manganese based formulation (Mn/BaCrO4/PbCrO4), a zirconium-nickel based formulation (Zr—Ni alloy/BaCrO4/KClO4), or what is termed a T-10 formulation (B/BaCrO4). While these delay compositions are effective and have been proven over years of use, the heavy metals, chromates, and perchlorates they contain are known to be toxic. Further, these prior art compositions are generally abrasive and it is difficult to press them to the desired high consolidated density required for an efficacious delay column with minimal void space, i.e. a delay column with mechanical integrity—without the use of excessive pressure and excessive wear of the production tooling.
Generally, the performance of pyrotechnic delays is sensitive to many factors, including component particle size, mixture stoichiometry, loading density and procedures, and the configuration of the housing of the device in which the pyrotechnic delay is used. Seemingly subtle differences in materials and methodology can cause significant changes to the resulting time delay. As a result, many military delay composition specifications are written broadly, and precise compositions are not assigned specific burning rates. Instead the key trends are mapped, so that the manufacturers can obtain the desired burning rate range, while empirically adjusting for specific loading procedures and lot-to-lot differences in raw materials.
Critically, military munition and signal time delays involve the packing/pressing of typically, a precisely, slow burning pyrotechnic material within a relatively short, small diameter channel or cavity, which is bored in a relatively large ruggedized housing, typically made from aluminum or zinc alloys. Such configurations are hereinafter referred to in this application as the pyrotechnic material being “pressed into a consolidated column” or simply “consolidated.” Aluminum and zinc alloys have very high thermal conductivity and thermal effusivity; where, thermal effusivity is a measure of a material's ability to transfer heat to its surroundings. Housings of such materials, with such high thermal conductivity and thermal effusivity—are effective heat sinks, which tend to quench slow burning delay compositions, halting the pyrotechnic reaction's propagation and thereby defeating the delay's functionality. Further, the HHS delay housing in particular is an especially large pancake-shaped aluminum disk with a very short and narrow channel—resulting in increased thermal losses, and further retarding the reaction's propagation, and thus further defeating the delay's functionality. The dimensions of the HHS delay housing may not be easily or practically changed to avoid these problems, as said housing doubles as a structural element of the HHS rocket, in addition to housing the pyrotechnic delay composition.
Prior published work, disclosed a new pyrotechnic delay system containing W, Sb2O3, KlO4, and calcium stearate for use in U.S. Army hand-held signals, that was functional in the large pancake-shaped HHS aluminum delay housing, and reliably gave desirable, relatively long, burning times of 5-6 s, 7-8.5 s/cm (wherein s/cm is an inverse burn rate, “IBR”—which is a preferred performance metric for slow-burning delay compositions, as subtle differences in low rates are easily distinguished when this reciprocal rate data is plotted). See, Poret et al., “Development and Performance of the W/Sb2O3/KlO4/lubricant Pyrotechnic Delay in the U.S. Army Hand-Held Signal”, Propellants, Explosives, Pyrotechnics, vol. 38, pp. 35-40, 2013. However, although the antimony oxide in this W/Sb2O3/KlO4/calcium stearate delay is arguably less hazardous than the BaCrO4 it replaced, such antimony compounds themselves are a health and environmental concern.
Therefore, there is a need in the art for a new, replacement pyrotechnic delay composition, which is nonabrasive, easily pressed and useful in military munitions and signals, that does not contain any environmentally hazardous materials, such as heavy metals, chromates, and perchlorates; that will burn hot enough, so as not to be quenched by heat losses to delay housings; while burning slow enough, to provide the required, relatively long, delay burn time (considering the very short burn path in the subject devices); and where the various variables that impact the delays performance are controllable.